The objective of this study is to improve our ability to characterize the toxicity of drugs and chemicals to neonates relative to adults, and to explore the role of lactation in the induction of neonatal toxicity. Lactation is evaluated as a source of exposure to chemicals secreted into the milk and as the nutritional source for the newborn. A study was completed with DDE which established and validated various techniques, and generated information about the effects of this chemical on various parameters of lactation, including milk production, milk composition, pup growth, and mammary gland integrity. Another study was completed which characterized the secretion of the liver carcinogen, dimethylnitrosamine, into milk and the subsequence genetic toxicity in the suckling neonates. A new project was begun to determine the toxicity of de(2-ethlhexyl) phthalate (DEHP) to suckling rat pups of different ages. Toxicity was evaluated in the livers and testes of the suckling rats. A second new study was begun to quantitate the transfer of the histamine H2-receptor antagonist, cimetidine, through the milk of lactating rats and to determine the antiandrogenic effects on the suckling male pups.